There is today ongoing work in the radio communication area for providing home and/or small area coverage for a limited number of users using a small base station, a Femto radio base station, commonly called a Femto NodeB for Wideband Code Division Multiple Access (WCDMA) or Femto eNodeB (Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) NodeB) for Long Term Evolution (LTE). Other common names are Home NodeB (HNB) for WCDMA or Home eNodeB (HeNB) for LTE.
This Femto radio base station would provide normal LTE/WCDMA coverage for the end users, a so called femto cell, and would be connected to the network using some kind of IP based transmission. One alternative is to use fixed broadband access, e.g. Digital Subscriber Line (xDSL) or Cable, to connect the Femto node to the network.
There are discussions to change the architecture for WCDMA from traditional architecture towards a flat architecture. However this does not change the system solution characteristics and behavior outlined below. For the flat architecture the Femto radio base station and Radio Network Controller (RNC) can be seen as logical entities in the Femto radio base station.
There are several ways to use the available frequencies between the femto layer and the traditional cellular deployment layer, which layer in this document is referred to as ‘macro’ layer even though it may comprise both macro, micro and pico cells. There are three Channel Deployment Scenarios (CDS) that are relevant, namely:                CDS1: One frequency used both by the Femto radio base stations and the Macro radio base station. This CDS has some severe interference problems that will make it hard to work.        CDS2: One dedicated frequency used only by the Femto radio base stations and at least another frequency used only by the Macro radio base stations. This CDS could be the preferred one if only technical arguments are taken into account. However, it is very unlikely that operators are willing to dedicate whole frequencies for the femto layer, mostly due to economical reasons.        CDS3: One frequency used both by the Femto radio base stations and the Macro radio base stations and at least another frequency used only by the Macro radio base stations.        
CDS3 is expected to be a common way for deployments. However, also this deployment can cause different types of interference in certain scenarios.
It is also assumed that Femto radio base stations belonging to one operator will typically be operating on one carrier/frequency.
A number Primary Scrambling Codes (PSC) is assumed to be allocated to the femto layer, e.g. ten PSCs. These PSCs are configured in the cell neighbor lists on the macro layer, and signaled to macro layer user equipments for their idle mode cell selection. One of these PSCs is automatically assigned to each Femto radio base station at start up.
A number of Location Areas (LA) is assumed to be allocated to the femto layer, e.g. ten LAs. One of these LAs is automatically assigned to each Femto radio base station at start up.
A Femto radio base station scans and reports found cells both macro and femto cells of any Radio Access Technology (RAT) to a network node, e.g. a logical RNC, concentrator node or another network node for example an Operation & Maintenance (O&M) node. The reported data includes, in the example of UMTS, for each detected UMTS Terrestrial Radio Access Network (UTRAN) cell:    1. PSC,    2. frequency e.g. UTRA Absolute Radio Frequency Channel Number (UARFCN),    3. relevant parts of system information, e.g. Location Area Identifier (LAI), Routing Area Code (RAC), Cell-ID, Common Pilot Channel (CPICH) output power, and    4. signal strength measurement report.
The network node, e.g. the (logical) RNC builds the neighbor list for a Femto radio base station, and includes the heard macro cells and the allocated femto PSCs in the neighbor list. The list is signaled to User Equipments (UEs) connected to the Femto radio base station for use in cell selection and cell reselection.
Femto radio base station ‘access control’ is based on a forbidden LAI list in the user equipment, i.e. LAI list in user equipment updated with the use of Location Update Reject.
For CDS3, the femto cells would for example reuse one of the two frequencies used in the macro cells. In a flat architecture for WCDMA, the radio base station and the RNC are collapsed to form one node, a Femto radio base station. In some documentation the resulting node is called NodeR&B (NodeB RNC and Radio base station) and also as a Combined RBS/RNC. However there might be a mix of traditional architecture and flat architecture nodes in a network in both the femto and macro layers.
Problems with existing solutions are described for a WCDMA embodiment. However, similar problems exist in the LTE network or other radio access technologies where user equipments of a Femto radio base station interfere with radio base stations or other Femto radio base stations. If frequencies are used according to CDS3 or similarly, as explained above, interference will occur in certain scenarios.